For example, Patent Literature 1 discloses a catalyst deterioration detecting device for an internal combustion engine. In this catalyst deterioration detecting device, an air/fuel ratio sensor is installed on the upstream of the catalyst, while an electromotive force-type oxygen sensor is installed on the downstream. In deterioration detection of a catalyst by this catalyst deterioration detecting device, an air/fuel ratio on the upstream of the catalyst is forcedly controlled so as to fluctuate between a predetermined rich air/fuel ratio and a lean air/fuel ratio. Then, a temporal value until output of the oxygen sensor on the downstream side changes from a lean output to a rich output or a temporal value until a lean output is detected from a rich output is detected in this control. In this deterioration detection of the catalyst, an oxygen storage capacity of the catalyst is calculated on the basis of such temporal value, and moreover, deterioration of the catalyst is determined on the basis of whether the calculated oxygen storage capacity is larger than a predetermined value or not.